There's several more factors to vtubers than that. It's more interesting if you like lore, it fits with the aesthetic of the rest of the video better, you don't need to get dressed or do your makeup, etc.

As far as privacy it doesn't necessarily apply because the famous ones almost all have previous careers as face streamers, and their fans know who they are since they use their real voices. But if you are doing it to hide yourself then it works, and it means that everyone else can interact with you and seem to be in the same world.